FIG. 8 is a block diagram illustrating a prior art compound semiconductor crystal growth apparatus for forming a thin film on a plurality of wafers by a low pressure chemical vapor deposition. More specifically, in this growth apparatus a chemical reaction of material gases under reduced pressure deposits a solid on the wafer.
More particularly, as illustrated in FIG. 8, a susceptor 1 holding a plurality of wafers 2 is placed in a reaction tube 3 having a cylindrical shape and, after both ends of the reaction tube 3 are closed with lids 4 and 5, a reactive gas (a mixture of material gases) is supplied to the reaction tube 3 via gas supply inlets 6a to 6c. Then, in a state where the pressure in the reaction tube 3 is reduced via an exhaust outlet 8 by an. exhaust pump 9, the inside of the reaction tube 3 is heated to a prescribed temperature by applying an electric current to a furnace 7, to deposit a crystalline film on the wafers 2 according to the chemical reaction of the material gas. Here, the flow rate of the material gas required for crystal growth, is controlled by mass flow controllers (hereinafter referred to as MFC) for supplying material gas 10a to 10c, and respective gases are introduced through material gas supply pipes 11a to 11c into a gas mixing pipe 12 for mixing those gases. The material gas mixed therein is introduced into the reaction tube 3 through gas distributing pipes 13a to 13c, in order to equalize the thickness, or control the composition of the crystal grown in the reaction tube 3.
By employing the above-described growth apparatus, gas distribution in the reaction tube is improved to a greater extent than that in a case where the reactive gas is provided via a single gas supply inlet from only one end of the reaction tube. However, the flow rates of the mixed material gas flowing through the respective gas distributing pipes are not made equal to each other by branching the mixed gas supply pipe into plural pipes and do not result in a thin film having a high quality.
In a low pressure chemical vapor deposition apparatus illustrated in FIG. 9, which is disclosed in Japanese Published Utility Model Application No. Hei.2-33260, according to an output voltage from a mass flow meter (hereinafter referred to as MFM) 15 placed in the gas mixing pipe 12, which output voltage is input to a gas controlling circuit 16, i.e., according to the flow rate of the mixed material gas flowing in the MFM 15, the flow rates of the mixed material gas flowing in the respective MFCs 14a, 14b and 14c which MFCs are respectively placed in the gas distributing pipes 13a, 13b and 13c, are controlled. The percentages of flow quantities for the respective MFCs 14a to 14c are kept at values which are preset at setting appliances 17a to 17c, controlled by the gas controlling circuit 16. As a result, the flow rates of the mixed material gas flowing through the respective gas distributing pipes 13a, 13b and 13c are automatically adjusted while keeping the percentages of flow quantities, and the flow rates of the mixed material gas flowing into the reaction tube 3 equalized.
When, between the material gas supplying MFCs 10a to 10c and the MFCs 14a to 14c, a difference in precision occurs due to errors in manufacture, deterioration over time or the like of the MFCs occurs, i.e., when though the gas controlling circuit 16 makes the flow rates of material gases flowing in the MFCs 14a to 14c equal to those flowing in the MFCs 10a to 10c, the total flow rates of the former MFCs 14a to 14c are smaller than those of the latter MFCs 10a to 10c. As a result, the mixed material gas is retarded in the gas mixing pipe 12 located between the MFCs 14a to 14c and the MFCs 10a to 10c, lowering the flow rate of the mixed material gas flowing in the MFM 15. In this structure, however, the flow rates of the reactive gas flowing in the MFCs 14a to 14c are lowered with a decrease in the flow rate of the material gas measured in the MFM 15, and further more material gases are retarded in the gas mixing pipe 12, further lowering the flow rates of the material gases, resulting in a vicious circle.
In the prior art compound semiconductor crystal growth apparatus constructed as described above, the material gas is retarded in the gas mixing pipe between the MFCs disposed respectively at prior and latter stage of the gas mixing pipe, due to errors in manufacture, deterioration over time or the like of the MFCs, and in the worst case, the supply of the mixed material gas stops.
FIG. 10 is a block diagram illustrating a prior art crystal growth apparatus, which is disclosed in Japanese Patent Application No. Hei.1-140712. In the figure, wafers 2 are disposed on a susceptor 31 having a rotation shaft 30. Flow rate variable valves 32a to 32d are disposed in gas distributing pipes 13. Prescribed material liquids are included in bubbling apparatus 33a to 33c and H.sub.2 gas as carrier gas is passed therethrough to generate desired material gases. Shut-off valves 35a to 35d are used for maintenance.
In this apparatus, after the material gases generated in the bubbling apparatus 33a to 33c are mixed in a gas mixing pipe 12, the ratio of flow quantities (distribution ratio) of the mixed material gases flowing through the gas distributing pipes 13 are controlled by the flow rate variable valves 32a to 32d having a prescribed degree of opening, and the mixed material gas is introduced into the reaction tube 3. The material gases introduced into the reaction tube 3 as described above react on the surface or in the vicinity of the wafers 2, growing desired crystals on the wafers 2. Thus, in this apparatus, the uniformity in the layer thickness and composition of the crystal layer grown on the wafer 2 is increased by adjusting the flow rates of the gases flowing through the gas distributing pipes 13. According to this prior art apparatus, however, the inherent nature of each material is not considered. Therefore, in processing a number of large sized wafers, it was difficult to control the uniformity of the layer thickness, the composition and the impurity addition amount of the grown crystal layer sufficiently over the whole surface of the wafer.
In the apparatus illustrated in FIG. 10, the material gases are generated using bubbling apparatus (containers of metal organic material). However, there is a problem that a small amount of metal organic material gas is not stably supplied in the conventional bubbling apparatus.
More particularly, as illustrated in FIG. 11, in conventional bubbling apparatus, by supplying a specific flow rate of a carrier gas, for example, H.sub.2, to a material container 53 (hereinafter also referred to as cylinder) from a pipe 51 for introducing carrier gas through an MFC 56 and passing the carrier gas through metal organic material 54, bubbles 55 are generated, and a material gas saturated with vapor from the metal organic material is exhausted from a pipe 52 for exhausting gas. At this time, by setting a control pressure of a pressure control gauge 57 placed in the gas exhausting pipe 52 at a prescribed value, a prescribed amount of material gas having a constant vapor pressure is obtained. However, in order to take out a small amount of material gas, a small amount of carrier gas must be introduced into the metal organic material. The bubbles generated at that time are not continuous bubbles but are intermittent and unstable because the quantity of introduced carrier gas is too small. Therefore, the amount of vapor of the material gas exhausted from the gas exhausting pipe 52 is also unstable, whereby the desired vapor flow from the metal organic material is not produced.
So as to solve the above-described problems, as illustrated in FIG. 12, an end of a carrier gas introducing pipe 51a is not immersed in the metal organic material 54. In this apparatus, a carrier gas supplied to the cylinder 53 does not generate any bubbles, takes in a saturated vapor at the surface of the metal organic material 54 in an equilibrium state, and is exhausted from the gas exhausting pipe 52 as a material gas. However, in this apparatus, as the metal organic material is consumed, the distance between the gas exhausting pipe 52 and the liquid level of the metal organic material 54 is varied. Therefore, the amount of vapor of the material gas taken in the gas exhausting pipe 52 varies with the consumption of the material, whereby a desired flow of material gas is not stably produced.